Re: [PATCH v4] xen/privcmd: fix error exit of privcmd_ioctl_dm_op()

[Juergen Gross <jgross@xxxxxxxx>]

On 26.08.22 11:08, Oleksandr Tyshchenko wrote:
> On 25.08.22 17:19, Juergen Gross wrote:
>
> Hello Juergen
>
> > The error exit of privcmd_ioctl_dm_op() is calling unlock_pages()
> > potentially with pages being NULL, leading to a NULL dereference.
> >
> > Additionally lock_pages() doesn't check for pin_user_pages_fast()
> > having been completely successful, resulting in potentially not
> > locking all pages into memory. This could result in sporadic failures
> > when using the related memory in user mode.
> >
> > Fix all of that by calling unlock_pages() always with the real number
> > of pinned pages, which will be zero in case pages being NULL, and by
> > checking the number of pages pinned by pin_user_pages_fast() matching
> > the expected number of pages.
> >
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Fixes: ab520be8cd5d ("xen/privcmd: Add IOCTL_PRIVCMD_DM_OP")
> > Reported-by: Rustam Subkhankulov <subkhankulov@xxxxxxxxx>
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>
> I haven't spotted any issues:
>
> Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>
>
> > ---
> > V2:
> > - use "pinned" as parameter for unlock_pages() (Jan Beulich)
> > - drop label "unlock" again (Jan Beulich)
> > - add check for complete success of pin_user_pages_fast()
> > V3:
> > - continue after partial success of pin_user_pages_fast() (Jan Beulich)
> > V4:
> > - fix case of multiple partial successes for one buffer (Jan Beulich)
> > ---
> >    drivers/xen/privcmd.c | 21 +++++++++++----------
> >    1 file changed, 11 insertions(+), 10 deletions(-)
> >
> > diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
> > index 3369734108af..e88e8f6f0a33 100644
> > --- a/drivers/xen/privcmd.c
> > +++ b/drivers/xen/privcmd.c
> > @@ -581,27 +581,30 @@ static int lock_pages(
> >         struct privcmd_dm_op_buf kbufs[], unsigned int num,
> >         struct page *pages[], unsigned int nr_pages, unsigned int *pinned)
> >    {
> > -       unsigned int i;
> > +       unsigned int i, off = 0;
> >    
> > -	for (i = 0; i < num; i++) {
> > +       for (i = 0; i < num; ) {
> >                 unsigned int requested;
> >                 int page_count;
> >    
> >    		requested = DIV_ROUND_UP(
> >                         offset_in_page(kbufs[i].uptr) + kbufs[i].size,
> > -                       PAGE_SIZE);
> > +                       PAGE_SIZE) - off;
> >                 if (requested > nr_pages)
> >                         return -ENOSPC;
> >    
> >    		page_count = pin_user_pages_fast(
> > -                       (unsigned long) kbufs[i].uptr,
> > +                       (unsigned long)kbufs[i].uptr + off * PAGE_SIZE,
> >                         requested, FOLL_WRITE, pages);
> > -               if (page_count < 0)
> > -                       return page_count;
> > +               if (page_count <= 0)
> > +                       return page_count ? : -EFAULT;
>
> [not related to the current patch]
>
> I just wonder, whether drivers/xen/gntdev.c:gntdev_get_page() really
> wants to gain the same check?
>
> index 59ffea800079..45e16031204d 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -740,8 +740,8 @@ static int gntdev_get_page(struct gntdev_copy_batch
> *batch, void __user *virt,
>           int ret;
>
>           ret = pin_user_pages_fast(addr, 1, batch->writeable ?
> FOLL_WRITE : 0, &page);
> -       if (ret < 0)
> -               return ret;
> +       if (ret <= 0)
> +               return ret ? : -EFAULT;
>
>           batch->pages[batch->nr_pages++] = page;
I don't think this is needed here, as pin_user_pages_fast() can't
return 0 when called for a single page.


Juergen

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